Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode (OLED) displays rely on passing current through a layer of organic material that glows in response to the current.
Many display systems, particularly for displays used in mobile applications such as tablet computers and smartphones, include a touchscreen that responds to commands provided by touching the touchscreen. A variety of touchscreen technologies are known, for example resistive, optical, acoustic, inductive, and capacitive.
Touchscreens are typically located over a display and use separate substrates and covers. Such an arrangement adds thickness and weight to a display system and absorbs light emitted by the display. In recent years, touchscreen components have been formed on display components, for example display covers, reducing the thickness and weight of the display system. For example, U.S. Pat. No. 8,243,027 describes a variety of touchscreen structures in a liquid crystal display having a backlight and color filters. U.S. Patent Application Publication No. 2010/0214247 discloses an array of touch elements including first and second electrodes forming in a plurality of two-dimensionally arranged capacitive sensing units in a layer.
In general, touch screens are either single-touch or multi-touch. Single-touch systems can detect only one touch at a time, for example most resistive touchscreens are of this type. Such screens are typically simple, fast, robust, easy to use with a variety of implements, and inexpensive to control and operate. In contrast, multi-touch touchscreens, for example self-capacitive or mutual-capacitive touch sensors, can detect multiple touch points on a screen at a time but are more limited in their touch modalities, for example limited to touches with a conductive stylus, such as a human finger. Such multi-touch systems use a matrix of touch sensors and are typically controlled using a sequential matrix scanning technique. For example, a mutual-capacitance touch system includes orthogonal arrays of horizontal and vertical overlapping electrodes. At every location where the horizontal and vertical electrodes overlap, a capacitor is formed, providing a capacitive touch sensor. A touch controller drives a row of touch sensors at a time and then reads a sense signal from each of the columns. Thus, only one row of sensors can be activated and read at a time. The touch controller sequentially drives successive rows to read back a signal from each touch sensor in the array. Because the rows of touch sensors are sequentially activated, as the touch sensor array grows larger and includes more rows, the rate at which the touch sensor array can be controlled decreases. This limits either the size (number of touch sensors in the touchscreen) or the scan rate at which touches can be detected, or both. Furthermore, touches in different rows are detected at different times. Such a control scheme is similar to the passive-matrix control used in small displays, for example small LCDs or OLED displays.
There remains a need, therefore, for alternative touchscreen structures that provide improved functionality and multi-touch capability and increased size and scan rates.